OUR FRESH-WATER POLYZOA. 
305 
Ascidige, or tunicated Molluscs, are now accordingly included 
with this latter group, and form together the section Mol- 
luscoida, (the other section being the Mollusca properly so 
called). A glance at the illustrations will show the general 
resemblance which exists between a Polyzoon and an Asci- 
dian, but for the determination of the precise homologous 
organs the reader must refer to Professor Allman’s valuable 
monograph.* 
In the Ascidian the respiration takes place by means of 
internal branchiae ; in the Polyzoon, by means of external 
branchiae forming around the mouth a corona of tentacles ciliated 
and protractile. They have both a distinct digestive tube, 
which turns on itself and is open at both extremities ; the 
nervous ganglion, though it does not occupy the same position 
in both animals, is of similar form and of a very rudimentary 
nature, and is regarded by Allman as a further instance of 
homology. f The most marked point of difference between the 
organization of a Polyzoon and a tunicated Mollusc consists 
in the latter animal being possessed of a heart and a vascular 
system, while in the Polyzoon no trace of anything of the kind 
exists. 
It will be well now to obtain some general conception of a 
Polyzoon, which we shall be readily able to do with the figures 
before us (Plate XIII., figs. 1 and 2), and from the following 
clear explanation of them by Professor Allman : — • 
“ Let us imagine an alimentary canal, consisting of oesophagus, stomach, 
and intestine, to be furnished at its origin with long ciliated tentacula, and to 
have a single nervous ganglion situated on one side of the oesophagus. Let 
us now suppose this canal to be bent back upon itself towards the side of the 
ganglion, so as to approximate the termination to the origin. Further, let us 
imagine the digestive tube thus constituted to be suspended in a fluid con- 
tained in a membranous sac, with two openings — one for the mouth and the 
other for the vent — the tentacula alone being external to the sac. Let us 
still further suppose the alimentary tube, by means of a system of muscles, 
to admit of being retracted or protruded according to the will of the animal, 
the retraction being accompanied by an invagination of the sac, so as partially 
or entirely to include the oral tentacula within it ; and if to these characters 
we add the presence of two sexual organs in the form of ovary and testis, 
occupying some portion of the interior of the sac, and the negative character 
of the absence of all vestige of a heart, we shall have, perhaps, as correct an 
idea — apart from all theoretical considerations of homology or derivation 
from an archetype — as can be conveyed of the essential structure of a 
Polyzoon in its simplest and most generalized condition. 
“ To give, however, more actuality to our ideal Polyzoon, we may bear in 
* “ On the Fresh- water Polyzoa.” ^ Pay Society, 1856. 
t i.e., Correspondence of parts, 
